linux-kernel - Re: [RFT PATCH v2 2/4] arm64: restore FPSIMD to default state for kernel and signal contexts

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Message-ID: <20131014151638.GI10491@mudshark.cambridge.arm.com>
Date:	Mon, 14 Oct 2013 16:16:38 +0100
From:	Will Deacon <will.deacon@....com>
To:	Jiang Liu <liuj97@...il.com>
Cc:	Catalin Marinas <Catalin.Marinas@....com>,
	Jiang Liu <jiang.liu@...wei.com>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFT PATCH v2 2/4] arm64: restore FPSIMD to default state for
 kernel and signal contexts

On Sun, Oct 13, 2013 at 03:20:18PM +0100, Jiang Liu wrote:
> From: Jiang Liu <jiang.liu@...wei.com>
> 
> Restore FPSIMD control and status registers to default values
> when creating new FPSIMD contexts for kernel context and reset
> FPSIMD status register when creating FPSIMD context for signal
> handling, otherwise the stale value in FPSIMD control and status
> registers may affect the new kernal or signal handling contexts.
> 
> Signed-off-by: Jiang Liu <jiang.liu@...wei.com>
> Cc: Jiang Liu <liuj97@...il.com>
> ---
>  arch/arm64/include/asm/fpsimd.h | 16 ++++++++++++++++
>  arch/arm64/kernel/fpsimd.c      | 11 +++++++++--
>  arch/arm64/kernel/signal.c      |  1 +
>  arch/arm64/kernel/signal32.c    |  1 +
>  4 files changed, 27 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/fpsimd.h b/arch/arm64/include/asm/fpsimd.h
> index c43b4ac..b2dc30f 100644
> --- a/arch/arm64/include/asm/fpsimd.h
> +++ b/arch/arm64/include/asm/fpsimd.h
> @@ -50,8 +50,24 @@ struct fpsimd_state {
>  #define VFP_STATE_SIZE		((32 * 8) + 4)
>  #endif
>  
> +#define	AARCH64_FPCR_DEFAULT_VAL	0
> +
>  struct task_struct;
>  
> +static inline void fpsimd_init_hw_state(void)
> +{
> +	int val = AARCH64_FPCR_DEFAULT_VAL;
> +
> +	asm ("msr fpcr, %x0\n"
> +	     "msr fpsr, xzr\n"
> +             : : "r"(val));
> +}
> +
> +static inline void fpsimd_clear_fpsr(void)
> +{
> +	asm ("msr fpsr, xzr\n");
> +}

You have pretty weak asm constraints here...

>  extern void fpsimd_save_state(struct fpsimd_state *state);
>  extern void fpsimd_load_state(struct fpsimd_state *state);
>  
> diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> index bb785d2..12a25e5 100644
> --- a/arch/arm64/kernel/fpsimd.c
> +++ b/arch/arm64/kernel/fpsimd.c
> @@ -80,9 +80,14 @@ void fpsimd_thread_switch(struct task_struct *next)
>  
>  void fpsimd_flush_thread(void)
>  {
> +	struct fpsimd_state *state = &current->thread.fpsimd_state;
> +
>  	preempt_disable();
> -	memset(&current->thread.fpsimd_state, 0, sizeof(struct fpsimd_state));
> -	fpsimd_load_state(&current->thread.fpsimd_state);
> +	memset(state, 0, sizeof(struct fpsimd_state));
> +#if (AARCH64_FPCR_DEFAULT_VAL != 0)
> +	state->fpcr = AARCH64_FPCR_DEFAULT_VAL;
> +#endif
> +	fpsimd_load_state(state);
>  	preempt_enable();
>  }
>  
> @@ -99,6 +104,8 @@ void kernel_neon_begin(void)
>  
>  	if (current->mm)
>  		fpsimd_save_state(&current->thread.fpsimd_state);
> +
> +	fpsimd_init_hw_state();
>  }
>  EXPORT_SYMBOL(kernel_neon_begin);
>  
> diff --git a/arch/arm64/kernel/signal.c b/arch/arm64/kernel/signal.c
> index 890a591..4ee231e 100644
> --- a/arch/arm64/kernel/signal.c
> +++ b/arch/arm64/kernel/signal.c
> @@ -52,6 +52,7 @@ static int preserve_fpsimd_context(struct fpsimd_context __user *ctx)
>  
>  	/* dump the hardware registers to the fpsimd_state structure */
>  	fpsimd_save_state(fpsimd);
> +	fpsimd_clear_fpsr();

... so I reckon GCC could reorder these two calls, resulting in corruption
of the saved state register.

Will
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